An investigation worth its salt: Targeting saline intrusion
Annual Conference
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Dunedin City Council (DCC) is in the process of delivering a $76 million upgrade of the Tahuna Wastewater Treatment Plant (WWTP) which will see 99% of the City’s reticulated domestic wastewater receiving secondary treatment through High Rate Activated Sludge (HRAS) and UV disinfection processes.
The viability of these treatment processes and subsequent ability to comply with new consent conditions is significantly threatened by rapid variation of influent salinity.
Recent completion of a $1.6m sewer structural rehabilitation of around 5,500m of sewer ranging from 150- 600mm in diameter produced significant a reduction in saline influence but the pilot plant was still experiencing unacceptable variation in the conductivity of influent, signalling the need for further investigation.
It was suspected that saline intrusion was occurring at unrepaired lateral junctions or through infiltration in the lateral themselves. A grab sample and flow monitoring programme was conducted in the rehabilitated area and subsequent CCTV inspection determined the exact location and nature of the intrusion.
The programme was then extended to investigate the entire coastal network.
This paper discusses the suite of challenges that an investigation of this nature presents with limited time windows for CCTV inspection and the countdown on for the completion of the Tahuna upgrade project; and reflects on the successes of a structured investigative approach.